1. Field of the Invention
The present invention relates to acid gas management schemes in the production of liquid fuels from hydrogen and carbon monoxide containing streams using light fossil fuel (for e.g. methane, natural gas, liquefied petroleum gas, naphtha), and solid feedstock such as biomass, coal, petroleum coke and the like.
2. Description of Related Art
Liquid fuels can be produced from solid feedstock materials by first gasifying the solid feedstock material to form hydrogen and carbon monoxide containing stream (gasifier syngas) that is further treated to form desired liquid fuel product.
Gasification of feedstocks such as coal or petroleum coke in an entrained flow gasifier generates a syngas with H2/CO ratio in the range of 0.4:1 to 0.8:1. CO2 content of the syngas is typically <10 vol. %. Gasification of feedstocks such as biomass in a fluidized bed gasifier typically generates a syngas with H2/CO ratio in the range of 0.8:1 to 1.2:1. On a dry basis, CO2 content of this syngas is in the range of 15-45 vol. %. Optimal H2/CO ratio for liquid fuel synthesis, e.g. Fischer-Tropsch (FT) liquids, methanol/gasoline, is around 2:1. Hence, the gasifier derived syngas typically undergoes the Water Gas Shift reaction to increase the H2/CO ratio. This further increases the CO2 content of the syngas. Higher CO2 in the syngas causes the H2 and CO partial pressures to be lower, thus resulting in lower per pass and overall yield of liquid fuel and a higher flow rate of tail gas from the liquid fuel synthesis unit, which is typically sent to a fuel header.
Hence, coal-to-liquids (CTL) and biomass-to-liquids (BTL) projects contain provisions for CO2 removal to low levels in order to reduce liquid fuels production system capital and operating costs.
To improve liquid fuel product yield several options are available to adjust the H2:CO ratio in the feed to the liquid fuel production unit. For example, the gasifier syngas can be mixed with a hydrogen and carbon monoxide containing gas that has a higher H2:CO ratio to form a mixture containing desirable H2:CO ratio; or hydrogen can be added to the gasifier syngas to increase H2:CO ratio, or some combination of these.
US Patent Application 2011/0218254 A1 teaches a method of using a light fossil fuel (e.g. natural gas) in an SMR/ATR/SMR-ATR system to produce a hydrogen rich syngas with H2:CO ratio greater than 2:1 and mixing that hydrogen rich syngas with the gasifier syngas as a means for adjusting the H2:CO ratio of the mixed syngas entering the unit that converts the mixed syngas to liquid. It also mentioned options for CO2 and H2S removal at different locations in the overall flow sheet without pointing out preferred scheme(s) or location(s).
U.S. Pat. No. 7,863,341 (assigned to Shell) discusses blending a H2-rich syngas from a light hydrocarbon feed with a gasifier derived syngas stream to adjust the H2:CO ratio of the syngas being fed to the liquids synthesis unit. Use of an acid gas removal system with one or more units to remove H2S and CO2 from the gasifier derived syngas is mentioned in general terms. No specifics are provided. It also mentions tail gas from the Fischer-Tropsch liquid fuels synthesis as a fuel source for the reforming unit generating the H2-rich syngas. Again, no specifics are provided.
U.S. Pat. No. 8,106,102 (assigned to SASOL and Haldor Topsoe) describes a Gas to Liquids process (no solid feedstock is utilized) where the tail gas from the liquids synthesis unit is reformed in a separate unit from the one that reforms the feed gas, such as natural gas.
Integrated Gasification Combined Cycle (IGCC) systems contain acid gas removal units that are typically designed for selective H2S removal. CO2 not removed by the acid gas removal (AGR) unit is rejected with the flue gas generated by combustion of gasifier derived syngas in the gas turbine section. However, IGCC systems primarily focus on making electricity and to a limited extent on liquid fuels production.
The need continues to exist for a liquid fuel production method and plant with lower capital and operating costs. It is an object of the present invention: to process liquid fuel product from hydrogen and carbon monoxide containing gases formed by converting solid feedstock using oxygen and by reforming and/or partial oxidation of light fossil fuels by utilizing                a smaller acid gas removal (AGR) system designed to remove all the H2S but only a portion of CO2 from the gasifier syngas. Typically, greater than 50%, and preferably greater than 65% of the CO2 in the gasifier syngas entering the AGR system leaves with the treated gasifier syngas        a lower cost amine system versus one that uses physical solvents for acid gas removal        a smaller sulfur recovery unit to recover sulfur in solid or liquid form from the acid gas stream leaving the acid gas removal system.        
Other objects and aspects of the present invention will become apparent to one of ordinary skill in the art upon review of the specification, drawings and claims appended hereto.